Reinforced plastics are combinations of fibers and polymeric binders, typically resins. The combination of the fibers and the binder form a composite material that can achieve a balance of material properties that are superior to the properties of either the fibers or the binder.
The combination of strong fibers and synthetic polymer binders to form reinforced plastics derives from several basic considerations of material science: the inherent strength of fine fibers, the wetting requirement for adhesion between the fibers and the binder, and the ease of the liquid-solid phase change of synthetic polymer binder. In a general sense, the polymeric binder matrix serves the purpose of a supporting medium surrounding each fiber and separating it from its neighbors, and stabilizing it against bending and buckling. These functions are best fulfilled when there is good adhesion between the fibers and the binder matrix. Adhesion can be fostered by utilizing a relatively low-viscosity liquid polymeric precursor to impregnate a reinforcing fiber material, followed by polymerization of the binder matrix. Adhesion can also be enhanced by plasma surface treatment of the fibers.
Two broad categories of polymeric materials have been typically utilized for the binder when preparing reinforced plastics. These two types are thermoplastic polymers, which generally melt in the range of 150° C. to 250° C. and readily solidify upon cooling, and thermosetting polymers which pass through a liquid phase just once during their life, while they are being polymerized and cross-linked into heat-infusible forms.
The two predominant types of fibers that have been used in reinforced plastics, considering all uses of composites, have been glass and cellulose fibers. Fibrous glass comprises well over 90% of the fibers used in reinforced plastics because it is inexpensive to produce and possesses high-strength, high-stiffness, low specific gravity, chemical resistance, and good insulating characteristics. In reinforced plastics, glass has been used in various forms. Fibrous glass can been chopped into short lengths (6-76 mm) and gathered into a felt or matte, resulting in a form that is easy to handle and low in cost. Previously, it has been observed that advantageous strength properties can be achieved in the final composite with nonwoven fabrics in which all the fibers are straight, continuous, and aligned parallel in a single direction.
In addition to glass and cellulose fibers, other types of fibers have also been used to reinforce plastic materials. The stiffest fibers known are composed of graphite, which theoretically can be almost five times more rigid than steel. However, despite much work over many years by many technical organizations, the cost of graphite fibers remains high. As a result, their use in composites is limited to applications that place a premium on weight savings: aircraft, missiles, sports equipment, etc.
In 1971, aromatic polyamide fibers became widely commercially available and are presently being used extensively in automotive tires and numerous aerospace structures. The aromatic polyamides are designated as “aramids” by the Federal Trade Commission, and that is the term used herein to refer to them. One specific aramid that has been widely used in many applications is referred to as KEVLAR. Discovered in 1965, KEVLAR is produced and marketed by DuPont.
In the stiffness range between glass and steel, aramids are lighter than glass, comparatively strong, much tougher, and absorb considerable energy before breaking, even under impact conditions. The fibers are highly crystalline and directional in character. KEVLAR fibers are known to have excellent resistance to flame and heat, organic solvents, fuels and lubricants, and they can be woven into fabric. Because of their strength and other properties, aramid fibers have been used in sports equipment, and in protective systems where ballistic stopping exploits their superior impact resistance.
Fibers of ultra-high strength polyethylene have been produced. Such fibers are available from Honeywell Advanced Fibers and Composites, Colonial Heights, Va., under the trademark SPECTRA, and Dutch State Mining Corporation (“DSM”). The fibers are made of extended chain polyethylene and have a low specific gravity of about 0.97, which is less than the specific gravity of fiberglass or aramid fibers.
Reinforced plastics are also used in medical and dental applications. As an example, U.S. Pat. No. 5,176,951, owned by the present applicant, discloses a method of reinforcing a dental appliance or prosthesis utilizing a reinforced plastic. The method disclosed therein includes the acts of applying to a resin portion of the dental appliance or prosthesis a lightweight, woven aramid or extended chain polyethylene fabric, and covering the fabric with more of the resin. Also disclosed are reinforcing materials (preferably a plasma-coated SPECTRA fabric), and dental appliances or prostheses reinforced by a lightweight, woven aramid or extended chain polyethylene fabric. While the methods and materials disclosed therein work well for their intended purposes, the methods and materials herein provide improvements.